Variable height boxes and background images.

hey I have come across a problem, what I want to do is have these colourful lines drop down and reach the boxes, after they have hit the boxes i want them to stop so that they do not continue. at current they are literally the background image of the div below.

Is it always going to be

Is it always going to be ordered like that getting longer as they go? If so, block the view of the background with an image that matches the black color of the background. If you need an example let me have access to your images.